1. Technical Field
This invention relates generally to radio frequency (RF) mixers and, in particular, to double balanced mixers.
2. Related Art
Even-order non-linearities in analog circuits result in undesirable even-order distortion that results in signal dependent direct current (DC) offsets. Since double balanced mixers featuring transistors (such as a Gilbert cell mixer) are differential, even-order harmonics appear as undesired effects of circuit mismatches. The circuit mismatches are seen in the shifting of the on-and-off cycle of the core transistors that results in even-order distortion and noticeable signal dependent DC offsets that can mask a desired DC signal. Certain kinds of transistor mismatches, such as area mismatch, result in signal dependent DC offsets. Double balanced mixers that perform direct conversion from a RF signal to a baseband signal are susceptible to the DC offset generated by the even-order distortions corrupting the desired output signal. Further, the DC offset is not dependent on frequency and creates additional problems in cellular telephony applications, such as GSM cellular applications, where blocking signals are present and may effectively mask the desired signal.
The degradation or loss of DC information during conversion to a DC signal is not a problem when intermediate steps of converting the RF signal to the baseband signal are used (usually due to AC coupling of the signal between stages). However, in a one-step conversion from the RF signal to the baseband signal, the DC offsets caused by the even-order distortions are a problem that is not easily resolved. When attempting to avoid a DC offset problem that adversely affects the signal, there are generally two approaches used to reduce the magnitude of the DC offset generated by the even order distortions as a result of the area mismatch. The first approach in suppressing the signal dependent DC offsets increases the core transistors"" size in order to reduce the area mismatch. The second approach utilizes large amounts of current in the local oscillator (LO) driver to increase the switching speed of the transistors. By increasing the switching speed, the transistors spend less time in the susceptible region (both transistors in the pair are on) where the area mismatch of the transistors adversely affects the output signal.
An increase in transistor size requires an increase in drive current without any positive effect on the switching speed. As drive current is increased, an undesirable reduction in battery time results. A reduction in battery time or life is undesirable in portable/mobile communication products. The second approach of increasing switching speed increases the current in the LO driver that is necessary to control the mixer core transistors and results in lower drive impedance. The higher bias current also adversely affects the battery life of portable/mobile communication products. Thus, either approach of suppressing even-order distortions requires large amounts of current that impact battery time. Therefore, what is needed in the art is a differential type mixer that can directly convert a RF signal to a baseband signal while avoiding a loss or degradation of the DC signal by reducing the even-order distortions without having to increase the transistor area or the bias current.
Broadly conceptualized, the invention is a bias voltage adjustment to one of the core transistors in a transistor pair of a double balanced mixer, such as a Gilbert cell mixer, to compensate for area mismatch between the mixer core transistors. The ratio of the area mismatch between any bipolar junction transistors (BJTs) is directly related to the base-emitter voltage of the mixer core transistors. Thus, an area mismatch is compensated for by adjusting the transistor bias voltage of one of the transistors in the mixer core. It can be shown that the matching between all four transistors in a double balanced mixer core is typically not required, but rather the matching between the two pairs of transistors that make up the mixer core is required in certain circumstances.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.